1. Field of the Invention
The present invention relates, in general, to communication systems, and, more particularly, to a programmable wavelength router for wavelength division multiplex (WDM) optical communication.
2. Statement of the Problem
Although optical fiber has very broad transmission bandwidth on the order of 10-20 THz, the system data rates transmitted over the fiber are presently limited to the modulation rate of the electrooptic modulators for single-channel communication using typical optical sources such as wavelength-tuned distributed feedback lasers. Information communication efficiency over an optical fiber transmission system can be increased by optical wavelength division multiplexing (WDM). WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information on an optical fiber. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome if these systems are to become commercially viable. For example, multiplexing, demultiplexing, and routing optical signals. The addition of the wavelength domain increases the complexity for network management because the processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels each defined by its own frequency spectrum into a single WDM signal. Demultiplexing is the opposite process in which a single WDM signal is decomposed into the individual channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from demultiplexing in that a router spatially separates the input optical channels into output ports and permutes these channels according to control signals to a desired coupling between an input channel and an output port.
One prior approach to wavelength routing has been to demultiplex the WDM signal into a number of component signals using a prism or diffraction grating. The component signals are each coupled to a plurality of 2.times.2 optical switches which are usually implemented as opto-mechanical switches. Optionally a signal to be added to the WDM signal is also coupled to one of the 2.times.2 switches. One output of each 2.times.2 optical switches coupled to a retained output multiplexer which combines the retained signals, and including the added signal, and couples them into a retained signal output port. A second signal for each 2.times.2 optical switch is coupled to a dropped signal multiplexer. By proper configuration of the optical switches, one signal can be coupled to the dropped signal output port, all the remaining signals pass through the retained signal output port. This structure is also known as a add-drop optical filter. The structure is complicated, relies on opto-mechanical switches, and interconnections tend to be difficult.
A "passive star" type of wavelength space switch has been used in some WDM networks, for example the LAMBDANET and the RAINBOW network. This passive star network has the broadest capability and the control structure and this implementation is notably simple. However, the splitting loss of the broadcast star can be quite high when the number of users is large. Also, the wavelength space switches used are based on tunable filters either Fabry-Perot type or acousto-optic based filters, which typically have narrow resonant peak or small side lobe compression ratio.
A third type of wavelength selectable space switch is shown in U.S. Pat. 5,488,500 issued to Glance. The Glance filter provides the advantage of arbitrary channel arrangement but suffers significant optical coupling loss because of the two array waveguide grading demultiplexers and two couplers used in the structure.
Another problem with prior approaches and with optical signal processing in general is high cross-talk between channels. Cross-talk occurs when optical energy from one channel causes a signal or noise to appear on another channel. Cross-talk must be minimized to provide reliable communication. Also, filters used in optical routing are often polarization dependent. The polarization dependency usually causes higher cross-talk as optical energy of particular polarization orientations may leak between channels or be difficult to spatially orient so that it can be properly launched into a selected output port. Similarly, optical filters provide imperfect pass band performance in that they provide too much attenuation or signal compression at side lobes of the pass band is not high enough. All of these features lead to imperfect or inefficient data communication using optical signals. What is needed is a routing structure that provides low cross-talk to eliminate the unnecessary interference from other channels in a large network, a flat pass band response in the optical spectrum of interest so that the wavelength router can tolerate small wavelength variations due to the laser wavelength drift, polarization insensitivity, and moderate to fast switching speed for network routing. Also, a router with low insertion loss is desirable so the router minimally impacts the network and limits the need for optical amplifiers.
3. Solution to the Problem
These and other problems of the prior art are solved by a digitally programmable wavelength router that can demultiplex any number of channels from a WDM signal and simultaneously spatially separate the channels and perform wavelength routing. Using optical switching elements to conventional logic level signals provides rapid switching and minimum power consumption during operation. Employing filters with wide flat band spectral response limits distortion and signal attenuation while providing desirable channel selectivity. Reliable low cross-talk routing is achieved with high immunity to polarization of the incoming WDM signal or any of the channels in the incoming WDM signal. By using a scaleable design, any number of channels can be placed in the WDM signal depending on the transmitter/detector technology and the optical fiber available.